Code division multiple access (CDMA) techniques have been employed in many digital wireless communication systems to permit a large number of system users to communicate with one another. Current CDMA networks were designed to carry only voice traffic and exhibit limited data rate variability. Thus, in conventional CDMA systems, users generally have similar bit rates and power levels.
CDMA systems treat each user as a separate signal, while treating the other users as either interference or noise. As the number of interfering users increases, the corresponding interferences add to cause degradation of performance, such as an increasing bit error rate (BER) or frame error rate. In a CDMA system, some users may be received at such high signal levels that a lower power user may be swamped out, referred to as the "near/far effect." In other words, users near the receiver are received at higher powers than those far away, and those further away suffer a degradation in performance. CDMA systems attempt to mitigate the near/far effect by employing power control to ensure that all users are received with equal power.
In theory, potentially significant capacity increases and near/far resistance can be achieved if the negative effects that each user has on each other can be cancelled. Of course, the performance improvements must be balanced with the attendant increases in system complexity. Multiuser detection and antenna array processing techniques have been suggested for use in CDMA systems to mitigate the negative effects that all users have on each other. Antenna array processing techniques mitigate the effects of different users by compensating for phase and delay effects. Multiuser detection algorithms, on the other hand, cancel interference in the time domain.
Since all the users in a conventional CDMA system generally have similar bit rates and power levels, and therefore contribute approximately the same degree of interference, the processing resources of an adaptive antenna array algorithm and a multiuser detection algorithm must be allocated among all the users. The complexity of both multiuser detection and antenna array processing is proportional to the number of users to be nulled. Thus, the computational burden of implementing an adaptive antenna array algorithm or a multiuser detection algorithm for all users is burdensome, and a number of suboptimal techniques have been implemented.
CDMA networks, however, are evolving to encompass a variety of multimedia applications, each having potentially different data rates. Thus, CDMA networks will be required to carry information associated with the various multimedia applications at various data rates, corresponding to the requirements of diverse wireless services demanded by customers. For example, CDMA networks will carry voice information characterized by low data rates and data information, including multimedia information, characterized by higher data rates. While interference in conventional (single-rate) CDMA systems is generally assumed to resemble Gaussian noise and be uniform in space, these assumptions do not hold in an environment having mixed rate traffic, where different users have different data rates.
Since the power level of a received signal increases in proportion to the bit rate (to achieve constant energy-per-bit, E.sub.b), signals having different power levels will be received on a channel having mixed rate traffic. The high-power users cause significant interference to the low-power users, reducing the overall system capacity (number of users). In a system with mixed-rate traffic, a small subset of high-powered users is responsible for a large fraction of the total received power.